Hey buddy, how have you been? I'm not really a big bar hopper, but I can tell you this: you must be 20 to buy alcohol in Japan. However, depending on where you go, it's not very difficult to get into bars and get a drink even if you're underage (well as long as you don't look 10...). On top of which, you're a foreigner, so you're unlikely to be asked for your ID in pubs, izakaya, or 12-seater bars...